Methods and systems for management of atrial retrograde conduction and pacemaker mediated tachyarrhythmia

ABSTRACT

Methods and systems for classifying cardiac responses to pacing stimulation and managing retrograde conduction and pacemaker mediated tachyarrhythmia are described. An atrial pacing pulse and a ventricular pacing pulse are delivered during a paced cardiac cycle. A post ventricular atrial refractory period (PVARP) is timed following the ventricular pacing pulse. The system determines if the atrial pacing pulse captures the atrium. An atrial depolarization occurring after the paced cardiac cycle is sensed. Retrograde management is initiated if the atrial pacing pulse did not capture the atrium and the atrial depolarization occurred during the PVARP. Pacemaker mediated tachyarrhythmia (PMT) is initiated if the atrial pacing pulse did not capture the atrium and the atrial depolarization did not occur during the PVARP.

RELATED PATENT DOCUMENT

This patent application is related to commonly owned U.S. patentapplication Ser. No. 11/601,217, entitled “DYNAMIC MORPHOLOGY BASEDATRIAL AUTOMATIC THRESHOLD,” filed concurrently herewith andincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devicesand, more particularly, to atrial pacing.

BACKGROUND OF THE INVENTION

When functioning normally, the heart produces rhythmic contractions andis capable of efficiently pumping blood throughout the body. However,due to disease or injury, the heart rhythm may become irregularresulting in diminished pumping efficiency.

Arrhythmia is a general term used to describe heart rhythmirregularities arising from a variety of physical conditions and diseaseprocesses. Cardiac rhythm management systems, such as implantablepacemakers and cardiac defibrillators, have been used as an effectivetreatment for patients with serious arrhythmias. These systems typicallyinclude circuitry to sense electrical signals from the heart and a pulsegenerator for delivering electrical stimulation pulses to the heart.Leads extending into the patient's heart are connected to electrodesthat contact the myocardium for sensing the heart's electrical signalsand for delivering stimulation pulses to the heart in accordance withvarious therapies.

Cardiac rhythm management systems operate to stimulate the heart tissueadjacent to the electrodes to produce a contraction of the tissue.Pacemakers are cardiac rhythm management systems that deliver a seriesof low energy pace pulses timed to assist the heart in producing acontractile rhythm that maintains cardiac pumping efficiency. Pacepulses may be intermittent or continuous, depending on the needs of thepatient. There exist a number of categories of pacemaker devices, withvarious modes for sensing and pacing one or more heart chambers.

When a pace pulse produces a contraction in the heart tissue, theelectrical cardiac signal following the contraction is denoted theevoked response (ER) signal. Superimposed on the evoked response signalis a signal associated with residual post pace polarization at theelectrode-tissue interface. The magnitude of the residual post pacepolarization signal, or pacing artifact, may be affected by a variety offactors including lead polarization, after-potential from the pacepulse, lead impedance, patient impedance, pace pulse width, and pacepulse amplitude, for example. The post pace polarization signal ispresent whether or not the pace captures the heart tissue.

A pace pulse must exceed a minimum energy value, or capture threshold,to produce a contraction. It is desirable for a pace pulse to havesufficient energy to stimulate capture of the heart without expendingenergy significantly in excess of the capture threshold. Thus, accuratedetermination of the capture threshold may be required for efficientpace energy management. If the pace pulse energy is too low, the pacepulses may not reliably produce a contractile response in the heart andmay result in ineffective pacing. If the pace pulse energy is too high,the patient may experience discomfort and the battery life of the devicewill be shorter.

Capture detection allows the cardiac rhythm management system to adjustthe energy level of pace pulses to correspond to the optimum energyexpenditure that reliably produces a contraction. Further, capturedetection allows the cardiac rhythm management system to initiate aback-up pulse at a higher energy level whenever a pace pulse does notproduce a contraction.

Retrograde conduction may occur, for example, when a depolarization waveinitiated in a ventricle by a pacing pulse or intrinsic activation ofthe ventricle travels back to the atrium producing a retrograde P-wave.Retrograde P-waves may inhibit effective atrial pacing. A pacing pulsedelivered to the atrium will not result in capture if the atrial tissueis refractory due to a retrograde P-wave. Further, retrograde conductionto the atrium may cause pacemaker mediated tachyarrhythmia (PMT).

There is a need for methods and systems that reliably determine if apacing pulse captures an atrium. There is a further need for methods andsystems that provide atrial retrograde management and PMT managementduring atrial pacing. The present invention fulfills these and otherneeds.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to methods and systemsfor retrograde management and PMT management during atrial pacing. Oneembodiment is directed to a method for initiating retrograde and PMTmanagement. An atrial pacing pulse and a ventricular pacing pulse aredelivered during a paced cardiac cycle. A post ventricular atrialrefractory period (PVARP) is timed following the ventricular pacingpulse. The system determines if the atrial pacing pulse captures theatrium. An atrial depolarization occurring after the paced cardiac cycleis sensed. Retrograde management is initiated if the atrial pacing pulsedid not capture the atrium and the atrial depolarization occurred duringthe PVARP. Pacemaker mediated tachyarrhythmia (PMT) is initiated if theatrial pacing pulse did not capture the atrium and the atrialdepolarization did not occur during the PVARP. For example the atrialdepolarization may comprise a retrogradely conducted depolarizationresponsive to the ventricular pacing pulse or a premature atrialcontraction.

According to one aspect of the invention, initiating retrogrademanagement involves modifying pacing for at least one pacing cycle. Inanother aspect, initiating the retrograde management involves delaying anext scheduled atrial pacing pulse following the paced cardiac cycle.According to yet another aspect, initiating the retrograde managementinvolves delaying a next scheduled atrial pacing pulse if the nextscheduled atrial pacing pulse is scheduled to occur when the atrialtissue is refractory. For example, the next scheduled atrial pacingpulse may be delayed until about 300 ms from the atrial depolarization.

In some implementations, initiating the PMT management involvesextending a subsequent PVARP of at least one pacing cycle occurringafter the paced cardiac cycle. In some implementations, initiating thePMT management comprises extending a subsequent PVARP of a next pacingcycle occurring after the paced cardiac cycle. For example, thesubsequent PVARP may be extended about 500 ms.

Another embodiment of the invention is directed to a cardiac rhythmmanagement system. The cardiac rhythm management system includes a pulsegenerator configured to deliver an atrial pacing pulse and a ventricularpacing pulse via electrodes coupled to a heart during a paced cardiaccycle. A post ventricular atrial refractory period (PVARP) timer times aPVARP following the ventricular pacing pulse. A capture detectordetermines if the atrial pacing pulse captures the atrium. A sensingsystem configured to senses an atrial depolarization that occurs afterthe paced cardiac cycle. A retrograde management module initiatesretrograde management pacing if the atrial pacing pulse did not capturethe atrium and the atrial depolarization occurred during the PVARP. Apacemaker mediated tachyarrhythmia (PMT) management module initiates PMTmanagement if the atrial pacing pulse did not capture the atrium and theatrial depolarization did not occur during the PVARP. The atrialdepolarization may comprise, for example a retrogradely conducteddepolarization responsive to the ventricular pacing pulse or a prematurecontraction.

In some implementations, the retrograde management module is configuredto delay a next scheduled atrial pacing pulse following the pacedcardiac cycle to initiate retrograde management. In someimplementations, the retrograde management module is configured to delaya next scheduled atrial pacing pulse if the next scheduled atrial pacingpulse is scheduled to occur when the tissue is refractory. For example,the retrograde management module is configured to delay the nextschedule atrial pacing pulse until about 300 ms from the atrialdepolarization.

The PMT management module is configured to extend a subsequent PVARP ofa next pacing cycle occurring after the paced cardiac cycle. Forexample, the subsequent PVARP may be extended to about 500 ms.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an implantable cardiac rhythm management (CRM) systemthat may be used in connection with atrial pacing methods in accordancewith embodiments of the invention;

FIG. 2 is a block diagram of a pacemaker that may be used to detectatrial capture and manage atrial retrograde conduction and PMT inaccordance with embodiments of the invention;

FIG. 3 is a flowchart illustrating a method for classifying the cardiacresponse to pacing that may be implemented by a CRM device in accordancewith embodiments of the invention;

FIG. 4A is a graph illustrating the morphology of a cardiac signalsensed following an atrial pace;

FIG. 4B is a diagram illustrating regions within the peak timinginterval used in pacing response discrimination in accordance withembodiments of the invention;

FIG. 5 is a flowchart illustrating step down capture threshold testingwith pacing response classification based on the regions depicted inFIG. 4B in accordance with embodiments of the invention;

FIG. 6 is a diagram illustrating regions used in pacing responsediscrimination in accordance with embodiments of the invention;

FIGS. 7A-7B illustrate a flowchart illustrating step down capturethreshold testing with pacing response classification based on theregions depicted in FIG. 6 in accordance with embodiments of theinvention;

FIG. 8 is a flowchart that illustrates an approach implementable in aCRM system for retrograde conduction management and PMT management inaccordance with embodiments of the invention;

FIG. 9 is a timing diagram illustrating a scenario where loss of capturemay be erroneously detected due to retrograde conduction;

FIG. 10 is a timing diagram illustrating retrograde management inaccordance with embodiments of the invention;

FIG. 11 is a timing diagram illustrating PMT caused by retrogradeconduction;

FIG. 12 is a timing diagram illustrating PMT management in accordancewith embodiments of the invention; and

FIG. 13 is a flowchart illustrating retrograde management and PMTmanagement in accordance with embodiments of the invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings forming a part hereof, and inwhich are shown, by way of illustration, various embodiments by whichthe invention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

Systems, devices or methods according to the present invention mayinclude one or more of the features, structures, methods, orcombinations thereof described herein. For example, a device or systemmay be implemented to include one or more of the advantageous featuresand/or processes described below. It is intended that such device orsystem need not include all of the features described herein, but may beimplemented to include selected features that provide for usefulstructures and/or functionality. Such a device or system may beimplemented to provide a variety of therapeutic or diagnostic functions.

After delivery of a pacing pulse to a heart chamber, various cardiacresponses to the pacing pulse are possible. In one scenario, the pacingpulse may generate a propagating wavefront of depolarization resultingin a contraction of the heart chamber. In this scenario, the pacingpulse is said to have captured the heart chamber. Capture of the heartchamber may occur if the pacing pulse has sufficient energy and isdelivered during a non-refractory period. If the pacing pulse does notproduce contraction of the chamber, the cardiac response is referred toas non-capture or loss of capture. Non-capture may occur, for example,if the pacing pulse energy is too low, and/or if the pacing pulse isdelivered during a refractory period of the cardiac tissue. Fusionoccurs when a depolarization initiated by a pace merges with anintrinsic depolarization.

Approaches for determining pacing response described herein rely onconsistency in the morphology of the cardiac signal sensed following apacing pulse to discriminate between noncapture, capture, and fusionresponses. The approaches described herein are particularly advantageouswhen used for atrial pacing response classification in conjunction withretrograde conduction management and/or PMT management. One or morefeatures of the sensed cardiac signal following pacing, e.g., peakmagnitude and peak timing, may be analyzed with respect to featurethresholds and/or timing intervals to determine the pacing response.

Pacing response classification such as by the methods described hereinmay be used with or without retrograde conduction management and/or PMTmanagement. If noncapture occurs, retrograde conduction from anintrinsic or paced ventricular depolarization may cause a falsenoncapture detection on the next pacing cycle. Retrograde conductionduring capture threshold testing, for example, may lead to erroneouscapture threshold determination. Retrograde conduction may also causeundesirable fast pacing, denoted pacemaker mediated tachyarrhythmia(PMT). Some embodiments described herein include methods and systemsthat provide for management of retrograde conduction and PMT.

By way of example, the processes of the present invention may be used incapture threshold testing to determine the optimal energy for pacing.Capture detection allows the cardiac rhythm management system to adjustthe energy level of pace pulses to correspond to the optimum energyexpenditure that reliably produces a contraction. Embodiments of thepresent invention are directed to methods and systems for pacingresponse classification to distinguish between capture, noncapture, andfusion based on atrial evoked response sensing in an implantablepacemaker, defibrillator or cardiac resynchronization therapy device.The pacing response classification processes described herein are basedon the use of timing windows and multiple amplitude thresholds totranslate atrial evoked response peak amplitude and timing informationinto capture, noncapture and fusion response classification.

Noncapture of the atrium by an atrial pace may allow retrogradeconduction to occur when a depolarization wave initiated in a ventricleby a pacing pulse or intrinsic activation of the ventricle travels backto the atrium producing a retrograde P-wave. A pacing pulse delivered tothe atrium will not result in capture if the atrial tissue is refractorydue to a retrograde P-wave. Retrograde P-waves may inhibit accuratedetermination of the capture threshold during a capture threshold test.Further, retrograde conduction to the atrium may cause pacemakermediated tachycardia (PMT). Embodiments of the invention are directed tomethods and systems for managing atrial retrograde conduction and PMT.

Those skilled in the art will appreciate that reference to a capturethreshold testing procedure indicates a method of determining thecapture threshold in one or more of the left atrium, right atrium, leftventricle, and right ventricle. In such a procedure, the pacemaker,automatically or upon command, initiates a search for the capturethreshold of the selected heart chamber. The capture threshold isdefined as the lowest pacing energy that consistently captures theheart.

In one example of an automatic capture threshold procedure, thepacemaker delivers a sequence of pacing pulses to the heart and detectsthe cardiac pacing responses to the pace pulses. The energy of thepacing pulses may be decreased in discrete steps until a predeterminednumber of noncapture responses occur. The pacemaker may increase thestimulation energy in discrete steps until a predetermined number ofcapture responses occur to confirm the capture threshold. A capturethreshold test may be performed using pacing response classification,retrograde management, and/or PMT management methods of the presentinvention.

Other procedures for implementing capture threshold testing may beutilized. In one example, the pacing energy may be increased in discretesteps until capture is detected. In another example, the pacing energymay be adjusted according to a binomial search pattern, or other searchpatterns.

Capture threshold determination is distinguishable from automaticcapture detection, a procedure that typically occurs on a beat-by-beatbasis during pacing. Automatic capture detection verifies that adelivered pace pulse results in a captured response. When a capturedresponse is not detected following a pace pulse, the pacemaker maydeliver a back up safety pace to ensure consistent pacing. If back uppacing is implemented, the back up pace may be delivered, for example,about 70-80 ms after the initial pace pulse. If a predetermined numberof pace pulses delivered during normal pacing do not produce a capturedresponse, the pacemaker may initiate a capture threshold test todetermine the capture threshold. Alternatively, if a predeterminednumber of pacing pulses do not produce a captured response, thepacemaker may adjust the pacing energy for the next pacing pulse. Pacingresponse classification, retrograde management, and/or PMT managementmay be implemented in conjunction with capture verification and/orcapture threshold testing using processes of the present invention.

Referring now to FIG. 1 of the drawings, there is shown a cardiac rhythmmanagement (CRM) system that may be used to implement discriminationbetween capture, noncapture, and fusion and/or to provide retrogrademanagement and/or PMT management in accordance with the approaches ofthe present invention. The CRM system in FIG. 1 includes a animplantable cardiac device (ICD) 100 such as a device incorporating thefunctions of a pacemaker, pacemaker/defibrillator, or cardiacresynchronization therapy (CRT) device, enclosed within a housing andcoupled to a lead system 102. The housing and/or header of the ICD 100may incorporate one or more can or indifferent electrodes 108, 109 usedto provide electrical stimulation energy to the heart and/or to sensecardiac electrical activity. The ICD 100 may utilize all or a portion ofthe pacemaker housing as a can electrode 108. The ICD 100 may include anindifferent electrode 109 positioned, for example, on the header or thehousing of the pacemaker 100. If the pacemaker 100 includes both a canelectrode 108 and an indifferent electrode 109, the electrodes 108, 109typically are electrically isolated from each other.

The lead system 102 is used to detect cardiac electrical signalsproduced by the heart and to provide electrical energy to the heartunder certain predetermined conditions to treat cardiac arrhythmias. Thelead system 102 may include one or more electrodes used for pacing,sensing, and/or defibrillation. In the embodiment shown in FIG. 1, thelead system 102 includes an intracardiac right ventricular (RV) leadsystem 104, an intracardiac right atrial (RA) lead system 105, and anintracardiac left ventricular (LV) lead system 106. An extracardiac leftatrial (LA) lead system 110 is employed in this example.

The CRM system illustrated in FIG. 1 is configured for biventricularand/or biatrial pacing. The lead system 102 illustrates one embodimentthat may be used in connection with the capture detection processesdescribed herein. Other leads and/or electrodes may additionally oralternatively be used. For example, the CRM system may pace multiplesites in one cardiac chamber via multiple electrodes within the chamber.This type of multisite pacing may be employed in one or more of theright atrium, left atrium, right ventricle or left ventricle. Multisitepacing in a chamber may be used for example, to increase the powerand/or synchrony of cardiac contractions of the paced chamber.

The lead system 102 may include intracardiac leads 104, 105, 106implanted in a human body with portions of the intracardiac leads 104,105, 106 inserted into a heart. The intracardiac leads 104, 105, 106include various electrodes positionable within the heart for sensingelectrical activity of the heart and for delivering electricalstimulation energy to the heart, for example, pacing pulses and/ordefibrillation shocks to treat various arrhythmias of the heart.

As illustrated in FIG. 1, the lead system 102 may include one or moreextracardiac leads 110 having electrodes 115, 118, e.g., epicardialelectrodes, positioned at locations outside the heart for sensing andpacing one or more heart chambers. In some configurations, theepicardial electrodes may be placed on or about the outside of the heartand/or embedded in the myocardium from locations outside the heart.

The right ventricular lead system 104 illustrated in FIG. 1 includes anSVC-coil 116, an RV-coil 114, an RV-ring electrode 111, and an RV-tipelectrode 112. The right ventricular lead system 104 extends through theright atrium and into the right ventricle. In particular, the RV-tipelectrode 112, RV-ring electrode 111, and RV-coil electrode 114 arepositioned at appropriate locations within the right ventricle forsensing and delivering electrical stimulation pulses to the heart. TheSVC-coil 116 is positioned at an appropriate location within the rightatrium chamber of the heart or a major vein leading to the right atrialchamber.

In one configuration, the RV-tip electrode 112 referenced to the canelectrode 108 may be used to implement unipolar pacing and/or sensing inthe right ventricle. Bipolar pacing and/or sensing in the rightventricle may be implemented using the RV-tip 112 and RV-ring 111electrodes. In yet another configuration, the RV-ring 111 electrode mayoptionally be omitted, and bipolar pacing and/or sensing may beaccomplished using the RV-tip electrode 112 and the RV-coil 114, forexample. The right ventricular lead system 104 may be configured as anintegrated bipolar pace/shock lead. The RV-coil 114 and the SVC-coil 116are defibrillation electrodes.

The left ventricular lead 106 includes an LV distal electrode 113 and anLV proximal electrode 117 located at appropriate locations in or aboutthe left ventricle for pacing and/or sensing the left ventricle. Theleft ventricular lead 106 may be guided into the right atrium of theheart via the superior vena cava. From the right atrium, the leftventricular lead 106 may be deployed into the coronary sinus ostium, theopening of the coronary sinus 150. The lead 106 may be guided throughthe coronary sinus 150 to a coronary vein of the left ventricle. Thisvein is used as an access pathway for leads to reach the surfaces of theleft ventricle which are not directly accessible from the right side ofthe heart. Lead placement for the left ventricular lead 106 may beachieved via subclavian vein access and a preformed guiding catheter forinsertion of the LV electrodes 113, 117 adjacent to the left ventricle.

Unipolar pacing and/or sensing in the left ventricle may be implemented,for example, using the LV distal electrode referenced to the canelectrode 108. The LV distal electrode 113 and the LV proximal electrode117 may be used together as bipolar sense and/or pace electrodes for theleft ventricle. The lead system 102 in conjunction with the ICD 100 mayprovide bradycardia pacing therapy to maintain a hemodynamicallysufficient heart rate. The left ventricular lead 106 and the rightventricular lead 104 and/or the right atrial lead and the left atriallead may be used to provide cardiac resynchronization therapy such thatthe ventricles and/or atria of the heart are paced substantiallysimultaneously or in phased sequence separated by an interventricular orinteratrial pacing delay, to provide enhanced cardiac pumping efficiencyfor patients suffering from congestive heart failure.

The right atrial lead 105 includes a RA-tip electrode 156 and an RA-ringelectrode 154 positioned at appropriate locations in the right atriumfor sensing and pacing the right atrium. In one configuration, theRA-tip 156 referenced to the can electrode 108, for example, may be usedto provide unipolar pacing and/or sensing in the right atrium. Inanother configuration, the RA-tip electrode 156 and the RA-ringelectrode 154 may be used to effect bipolar pacing and/or sensing.

Referring now to FIG. 2A, there is shown a block diagram of anembodiment of a CRM system 200 suitable for implementing atrial pacingwith cardiac response classification, retrograde management and/or PMTmanagement according to the approaches of the present invention.

The CRM system 200 includes a control processor 240 capable ofcontrolling the delivery of pacing pulses or defibrillation shocks tothe right ventricle, left ventricle, right atrium and/or left atrium.The pacing pulse generator 230 is configured to generate pacing pulsesfor treating bradyarrhythmia, for example, or for synchronizing thecontractions of contralateral heart chambers using biatrial and/orbiventricular pacing.

The control processor 240 may include an arrhythmia detector thatoperates to detect atrial or ventricular tachyarrhythmia orfibrillation. Under control of the control processor 240, thecardioversion/defibrillation pulse generator 235 is capable ofgenerating high energy shocks to terminate the detected tachyarrhythmiaepisodes.

The pacing pulses and/or defibrillation shocks are delivered viamultiple cardiac electrodes 205 electrically coupled to a heart anddisposed at multiple locations within, on, or about the heart. One ormore electrodes 205 may be disposed in, on, or about a heart chamber orat multiple sites of the heart chamber. The electrodes 205 are coupledto switch matrix 225 circuitry that is used to selectively couple theelectrodes 205 to the sense circuitry 210 and the therapy pulsegenerators 230, 235.

The CRM system 200 includes a pacing response classification (PRC)processor 215. In some embodiments, the PRC processor 215 is configuredto discriminate between capture and non-capture. In some embodiments,the PRC processor is configured to discriminate between capture,noncapture and fusion in accordance with embodiments described herein.Pacing response classification is implemented by the PRC processor 215for capture threshold testing and/or capture verification duringtherapeutic pacing. The PRC processor 215 is configured to determinevarious thresholds and intervals useful in the analysis of signals todetermine the pacing response. For example, the PRC processor 215 maydetermine one or more of a pacing threshold interval (PTI), a pacingartifact threshold (PAT), and/or a capture detection threshold (CDT).Discrimination between capture, noncapture, and fusion is performed bythe PRC processor 215 based on comparison of a cardiac signal sensedfollowing a pacing pulse to one or more of the intervals or thresholds.

The control processor 240 includes a capture threshold module 243 thatcontrols the operation of capture threshold testing. The controlprocessor 240 includes an atrial refractory period timer 241 for timingatrial refractory (ARP) and/or post ventricular atrial refractory period(PVARP) intervals following atrial and/or ventricular paces and/orsenses. The control processor 200 may optionally include a retrogrademanagement module 242 configured to control pacing during retrogrademanagement pacing cycles. The control processor 240 may optionallyinclude PMT management module 244 configured to control pacing duringPMT management pacing cycles.

The capture threshold module 243 controls the delivery of paces by thepacing therapy pulse generator 230 during therapeutic pacing and duringcapture threshold testing. To determine the capture threshold, thecapture threshold module 243 may control the delivery of a sequence ofpacing pulses that incrementally step down or step up the pacing energyuntil a capture threshold is determined. Prior to beginning the capturethreshold test, the capture threshold module 243 may control pacingduring an initialization procedure. During the initialization procedure,the PRC processor 215 operates to determine thresholds and intervalsdescribed herein that are useful in cardiac pacing responseclassification. The thresholds and intervals determined in theinitiation procedure are then used to determine the pacing responses tothe threshold test paces.

The CRM system 200 is typically powered by an electrochemical battery(not shown). A memory 245 stores data and program commands used toimplement the pacing response classification, retrograde managementand/or PMT management approaches described herein along with otherfeatures. Data and program commands may be transferred between the CRMsystem 200 and a patient-external device 255 via telemetry-basedcommunications circuitry 250.

FIG. 2 shows a CRM system 200 divided into functional blocks. It isunderstood by those skilled in the art that there exist many possibleconfigurations in which these functional blocks can be arranged. Theexample depicted in FIG. 2 is one possible functional arrangement. Otherarrangements are also possible. For example, more, fewer or differentfunctional blocks may be used to describe a cardiac system suitable forimplementing the processes of the present invention. In addition,although the CRM system 200 depicted in FIG. 2 contemplates the use of aprogrammable microprocessor-based logic circuit, other circuitimplementations may be utilized.

FIG. 3 is a flowchart illustrating a method for classifying the cardiacresponse to pacing a heart chamber, such as an atrial heart chamber,that may be implemented by a CRM device in accordance with embodimentsof the invention. The time between a delivered atrial pace and theevoked response signal peak is substantially consistent. A peak timinginterval (PTI) may be established for examining the cardiac signal todetermine the pacing response. The magnitude of the peak may be used toclassify the pacing response.

A method in accordance with one embodiment involves discriminatingbetween capture, noncapture, and fusion based on comparison of a sensedcardiac signal peak to a capture detection threshold (CDT), a pacingartifact threshold (PAT), and a peak timing interval (PTI). The PAT isdetermined 310 based on peak values of atrial signals of one or morenoncaptured cardiac cycles, e.g., about 2 to about 4 cardiac cycles. Thesignals used to determine the PAT may be sensed following sub-capturethreshold paces following a capture threshold test, for example. Thesensed atrial signals associated with noncapture are pacing artifactsignals that have a morphology exhibiting a pacing artifact without theevoked response morphology produced by capture. In variousimplementations, the PAT may be based on or a combination of the peakvalues of the signals associated with noncapture. For example, the PATmay be based on the peak magnitude of a most recent cardiac signalassociated with capture, the largest one or more peak magnitudes of thesignals associated with noncapture, a median value of the magnitudes ofthe signals associated with noncapture, a mean value of the magnitudesof the signals associated with noncapture, a weighted average of themagnitudes of the signals associated with noncapture, or othercombination of the peak magnitudes of the signals associated withnoncapture. The PAT may include an offset to take into account thevariability of the peak magnitudes of the signals associated withnoncapture. In one example, the PAT is set to a percentage, such asabout 150%, of the peak magnitude of a most recent signal associatedwith noncapture.

A capture detection threshold (CDT) is determined 320 based on peakvalues of one or more evoked response signals, e.g., about 5 to about 10signals, detected during one or more captured cardiac cycles. Thesignals used to determine the CDT follow supra capture threshold paces.The signals associated with capture exhibit a morphology that includesan evoked response signal having a superimposed pacing artifact signal.Similarly to the PAT determination described above, the CDT may be basedon or a combination of the peak values of the signals associated withcapture. The CDT may be based on a most recent peak magnitude of asignal associated with capture, the largest one or more peak magnitudesof the signals associated with capture, a median value of the peakmagnitudes of the signals associated with capture, a mean value of thepeak magnitudes of the signals associated with capture, or a weightedaverage of the peak magnitudes of the signals associated with capture,or other combination of the peak magnitudes of the signals associatedwith capture. The CDT may include an offset to take into account thevariability of the peak magnitudes of the signals associated withcapture. The use of a weighted average for the CDT provides such anoffset, for example. In one embodiment, the CDT is set to a percentageof an average, e.g., about 70% of evoked response peak magnitudes.

A peak time interval (PTI) associated with an expected timing of theevoked response signal peak is used in conjunction with the PAT and theCDT. Discrimination between capture, noncapture, and fusion is based oncomparison of the magnitude of the cardiac signal peak relative to thePAT and CDT and comparison of the timing of the cardiac signal peakrelative to the PTI.

The PTI is determined based on the timing of peak values of one or moreevoked response signals detected during one or more captured cardiaccycles. The signals used to determine the PTI follow supra capturethreshold paces. The PTI may be determined based on the variability ofthe peak timing of the signals associated with capture, for example. Atypical value of the PTI is about 9 ms, for example. The PTI may bebased on a median value of the peak timings of the signals associatedwith capture, a mean value of the peak timings of the signals associatedwith capture, or a weighted average of the peak timings of the signalsassociated with capture, or other combination of the peak timings of thesignals associated with capture. The PTI may include predeterminedinterval offsets on either side of a most recent, average, mean, ormedian timing value, for example, where the interval offsets take intoaccount the variability of the peak timing of signals associated withcapture.

A cardiac signal following a pacing pulse of a cardiac cycle subsequentto the noncaptured cardiac cycles and the captured cardiac cycles issensed 330. A peak value of the sensed cardiac signal falling within thePTI is compared 340 to the PAT and to the CDT. The device discriminates350 between capture, noncapture, and fusion based on the comparison. Ifthe signal peak is less than the PAT, then the pacing response isdetermined to be noncapture. If the signal peak is greater than the CDT,then the pacing response is determined to be capture. If the signal peakfalls between the PAT and the CDT, then the pacing response may benoncapture or may be fusion.

FIG. 4A is a graph illustrating the morphology of a captured responsesignal 410 sensed following an atrial pace (Ap). The peak 411 of thecardiac signal 410 depicted in FIG. 4A has a magnitude (i.e., absolutevalue) larger than the PAT 430 and the CDT 440.

FIG. 4B is a diagram illustrating regions used in pacing responsediscrimination in accordance with one embodiment. FIG. 4B shows the PAT430, the CDT 440, and the PTI 450 which define regions 460, 470, and 480respectively associated with noncapture (NC), capture, and bothnoncapture and fusion. If the peak of a cardiac signal following pacingfalls within a particular region 460, 470, 480, then the cardiac pacingresponse is classified as likely to be the type of response or responsesassociated with the region 460, 470, 480.

In one implementation, a counter for a particular type of response isincremented each time a peak falls within a region associated with theparticular type of response. The counter increments may be integer orfractional increments. The counter increments may be based on thelikelihood that a particular type of pacing response has occurred. Forexample, region 470 is associated with both noncapture and fusion.However, it may be more likely that a peak falling in region 470 isfusion rather than noncapture. If a peak falls within region 470, thefusion counter may be incremented by 1 and the noncapture counter may beincremented by ½. In some scenarios, confirmation that a particularpacing response has been occurring may require several cardiac cycles.For example, confirmation of the particular type of pacing response mayoccur if a counter for the particular type of pacing response reaches apredetermined value.

FIG. 5 is a flowchart illustrating step down capture threshold testingwith pacing response classification based on the regions depicted inFIG. 4B. The approaches of the present invention may advantageously beused in connection with capture threshold testing. Prior to beginningthe step down test, the CDT and/or PTI are initialized 505 based on thepeak magnitudes of signals sensed following delivery of a series ofsupra capture threshold paces.

In one embodiment, the PAT is initialized to a predetermined value, suchas about 0.3 mV prior to the capture threshold test. The CDT and PTI areinitialized based on measured values of the peak magnitude and peaktiming of captured signals Initialization of the CDT and PTI prior tothe test based on measured values provides patient specific values,enhancing the accuracy of capture testing. In addition, one or more ofthese parameters may be modified during and/or after the capturethreshold test based on most recent peak timing and peak magnitudevalues to further enhance the test accuracy.

The pacing energy and the pacing rate are initialized 510 for the test.A pace is delivered 515 and the cardiac signal following the pace issensed 515. The peak magnitude (P_(M)) of the cardiac signal isdetermined 520. If the peak magnitude is less than or equal to 525 thePAT, then the pace did not capture the chamber and the noncapturecounter is incremented 530. If the peak timing is within the PTI and thepeak magnitude is greater than the PAT but is less than or equal to theCDT 535, then the pacing response may be noncapture or may be fusion.Both the noncapture counter and the fusion counter are incremented 540.If the peak timing is within the PTI and the peak magnitude is greaterthan the CDT, then the pacing response is 545 capture and the capturecounter is incremented 550. Otherwise, the response is determined 546 tobe fusion.

The amounts that the counters for each type of response are incrementedmay be integer or fractional amounts. In some implementations, theamount that a particular counter is incremented is associated with thelikelihood that the type of pacing response occurred. For example, ifthe peak magnitude falls between the PAT and the CDT, fusion is morelikely than noncapture. In this scenario, the fusion counter may beincremented by 1 and the noncapture counter incremented by ½.

If the noncapture counter reaches 555, 556 a predetermined value, e.g.,about 2, for paces having the same energy, then loss of capture isconfirmed and the capture threshold is determined 560. If the fusioncounter reaches a predetermined value, e.g., about 5, then the pacingrate is increased 570 to decrease the occurrence of fusion beats. If thecapture counter reaches 575 a predetermined value, e.g., about 3 forpaces having the same energy, then the pacing energy is stepped down 580and the test continues until the capture threshold is determined 560.

In some implementations, the PAT, CDT, and PTI may be initialized beforethe test and/or one or more of these parameters may be modified duringthe test, such as during every cardiac cycle, and/or may be modifiedafter the test. The PAT may be re-initialized in the case of certainfailures.

In one example, the peak timing and/or peak magnitude may be determinedfor the cardiac signal of each beat. The peak timing and/or peakmagnitude may be combined with peak timings and magnitudes of one ormore previous beats to dynamically modify the PTI and CDT during thetest. Modifying the PTI and/or CDT during the test may be used to adaptto changing patient conditions, providing more accurate values for theseparameters. The PAT may be modified after the test based on one or morenoncaptured signals, detected after the capture threshold is determined.Modifying the PAT based on a particular patient's pacing artifactmorphology allows for adaptation to changing patient conditions overtime and provides more accurate pacing response classification.

FIG. 6 is a diagram illustrating regions used in pacing responsediscrimination in accordance with another embodiment. Fusion beatsusually exhibit large variations in peak timing of the cardiac signalwhen compared to captured beats. Regions corresponding to time intervalsbefore and/or after the PTI may be used for fusion discrimination. FIG.6 shows the PAT 630, the CDT 640, and the PTI 650 which define regions661-663 associated with noncapture, region 670 associated withnoncapture and fusion, region 680 associated with capture, and regions691-694 associated with fusion. If the peak of a cardiac signalfollowing pacing falls within a particular region, then the cardiacpacing response is likely to be the type of response associated with theregion.

As previously described, a counter for a particular type of response maybe incremented each time a peak falls within a region associated withthe particular type of response. The increments may be integer orfractional increments. The counter increments may be based on thelikelihood that a particular type of pacing response has occurred. Forexample, region 670 is associated with both noncapture and fusion.However, it may be more likely that a peak falling in region 670 isfusion rather than noncapture. If a peak falls within region 670, thefusion counter may be incremented by 1 and the noncapture may beincremented by ½. In some scenarios, confirmation that a particularpacing response has been occurring may require several cardiac cycles.For example, confirmation of the particular type of pacing response mayoccur if a counter for the particular type of pacing response reaches apredetermined value.

FIGS. 7A-7B illustrate a flowchart illustrating step down capturethreshold testing with pacing response classification based on theregions depicted in FIG. 6. Prior to beginning the step down test, thePAT, CDT, and PTI are initialized 705. The PAT is initialized to apredetermined value. The CDT is initialized based peak magnitudes,P_(m), of signals sensed following delivery of supra capture thresholdpaces. PTI is initialized based on the peak timing, P_(t), of signalssensed following delivery of supra capture threshold paces. The pacingenergy and the pacing rate are initialized 710 for the test.

A pace is delivered 715 and the cardiac signal following the pace issensed 715. The peak magnitude, P_(m), of the cardiac signal isdetermined 720. If the peak magnitude is less than or equal to 725 thePAT, then the pace did not capture the chamber and the noncapturecounter is incremented 730. If the peak magnitude is greater than thePAT and the timing of the peak does not fall 735 within the PTI, thenthe pacing response is likely to be fusion and the fusion counter isincremented 740.

If the timing of the peak falls within the PTI and the peak magnitude isgreater than the PAT and less than or equal to 745 the CDT, then thepacing response may be fusion or noncapture. The fusion counter and thenoncapture counter are incremented 750. If the peak magnitude is greaterthan the CDT, then the pacing response is 755 capture and the capturecounter is incremented 760.

As previously described, the amounts that the counters for each type ofresponse are incremented may be integer or fractional amounts. In someimplementations, the amount that a particular counter is incremented isassociated with the likelihood that the type of pacing responseoccurred. For example, if the peak magnitude falls between the PAT andthe CDT, fusion is more likely than noncapture. In this scenario, thefusion counter may be incremented by 1 and the noncapture counterincremented by ½.

If the noncapture counter reaches 770, 771 a predetermined value, e.g.,about 2, for paces having the same energy, then loss of capture isconfirmed and the capture threshold is determined 775. If the fusioncounter reaches 780 a predetermined value, e.g., about 5, then thepacing rate is increased 785 to avoid the occurrence of fusion beats. Ifthe capture counter reaches 790 a predetermined value, e.g., about 3 forpaces having the same energy, then the pacing energy is stepped down 795and the test continues until the capture threshold is determined 775.Following the capture threshold test, the PAT may be updated based onthe peak magnitude of one or more noncaptured signals

During pacing, if noncapture occurs, retrograde conduction from anintrinsic or paced ventricular depolarization may cause a falsenoncapture detection on the next pacing cycle. Retrograde conductionduring capture threshold testing, for example, may lead to erroneouscapture threshold determination. Retrograde conduction may also causeundesirable fast pacing, denoted pacemaker mediated tachyarrhythmia(PMT). Some embodiments described herein include methods and systemsthat provide for management of retrograde conduction and PMT.

The flowchart of FIG. 8 illustrates an approach implementable in a CRMsystem for retrograde conduction management and PMT management inaccordance with embodiments of the invention. An atrial pace and aventricular pace are delivered 810 during a cardiac cycle. A postventricular atrial refractory period (PVARP) is timed 820 following theventricular pace. The CRM system determines 830 if the atrial pacecaptured the atrium. In some embodiments, capture may be detected basedon comparison of peak magnitude and timing of the cardiac signalfollowing pacing to the CDT, PAT, and PTI as described above. In otherembodiments, capture may be determined using other capture detectionmethods known in the art.

If capture occurs, the depolarization associated with capture causestissue refractoriness, making retrograde conduction unlikely. Ifnoncapture occurs, the atrial tissue is not refractory after the paceand the ventricular depolarization may conduct retrogradely to theatrium. The system senses 840 an atrial depolarization following thepacing cycle indicative of retrograde conduction. Retrograde managementis initiated 850 if the atrial pacing pulse did not capture and anatrial depolarization is sensed during the PVARP. PMT management isinitiated 860 if the atrial pacing pulse did not capture and an atrialdepolarization is sensed after the PVARP.

The timing diagram of FIG. 9 illustrates a scenario where noncapture iserroneously detected due to retrograde conduction. A noncaptured atrialpace 910 and a ventricular pace 930 are delivered during a first cardiaccycle. A PVARP 920 is timed is following the ventricular pace 930. Inthis cycle, the atrial pace may be accurately detected as noncapture.However, confirmation of the loss of capture during a capture thresholdtest typically requires more than one noncaptured pace, such as severalnoncaptured paces detected consecutively or within a short period oftime. If a noncapture event was caused by transient effects, such asnoise, rather than by the decrease in the pacing energy, then loss ofcapture would not be confirmed because subsequent paces would becaptured and the test would continue. However, a pattern of retrogradeconduction may be initiated by the noncaptured pace, causing a singlenoncaptured pace to result in an erroneous loss of capture confirmationas described below.

Because the atrial pace 910 did not produce capture, the depolarizationcaused by the ventricular pace causes retrograde conduction to theatrium. The retrograde conduction produces an atrial depolarization 940causing the atrial tissue to become refractory. The atrialdepolarization 940 does not initiate a new pacing cycle because isoccurs during PVARP 920. The atrial pace 911 for the next cycle isdelivered during the tissue refractory period 950. Because the atrialpace 911 is delivered while the tissue is refractory, the pace 911 isdetected as noncapture. During a capture threshold test, the noncapturedatrial pace causes a false detection of noncapture because thenoncapture is the result of tissue refractoriness following retrogradeconduction rather than the change in the pacing energy level. Noncaptureof the atrial pace 911 during the second cardiac cycle again causesretrograde conduction, an atrial depolarization 941, and tissuerefractoriness. The pattern of false noncapture detection and retrogradeconduction may continue resulting in a confirmation of loss of captureand an erroneous capture threshold measurement.

The timing diagram illustrated in FIG. 10 illustrates retrogrademanagement in accordance with embodiments of the invention. The atrialpace 1010 of the first cardiac cycle is noncaptured. The ventricularpace 1020 of the first cardiac cycle causes retrograde conduction to theatrium. An atrial depolarization 1040 produced by the retrogradeconduction causes the atrial tissue to become refractory during a tissuerefractory period 1050. The atrial depolarization 1040 does not initiatea new pacing cycle because the atrial depolarization occurs during PVARP1020. The CRM system senses the atrial depolarization 1040 that occursduring the PVARP 1020. The next scheduled atrial pace 1011 for the cyclefollowing the retrograde conduction is delayed until after the tissuerefractory period 1050 ends. Typically the period 1050 of tissuerefractoriness lasts less than 300 ms after the depolarization 1040 issensed, for example. Therefore, the next scheduled atrial pace 1011 inthis example is delayed until about 300 ms following the atrialdepolarization 1040.

The delayed pace 1011 is correctly classified as a captured pace. Thethird cardiac cycle includes an atrial pace 1012 and a ventricular pace1032 that are delivered at the scheduled time.

FIGS. 9 and 10 above illustrate retrograde conduction when theretrograde atrial depolarization occurs during PVARP. In this scenario,the retrograde atrial depolarization does not initiate a new pacingcycle. Retrograde conduction producing atrial depolarizations that occurafter PVARP has expired may result in PMT. PMT caused by retrogradeconduction is illustrated in the timing diagram of FIG. 11. The firstcardiac cycle includes a noncaptured atrial pace 1110 and a capturedventricular pace 1130. PVARP 1120 is timed following the ventricularpace 1130, 1131 for each cycle. The noncaptured atrial pace 1110 in thefirst cycle allows the depolarization initiated by the capturedventricular pace 1130 of the first cycle to conduct retrogradely to theatrium. An atrial depolarization caused by the retrograde conductioncauses a nonrefractory atrial sense. Because the atrial sense 1141occurs after expiration of PVARP 1120 (i.e., is a nonrefractory sense),the CRM system initiates a pacing cycle in the second cardiac cyclewhich is abnormally fast. The pattern of fast ventricular paces andretrograde atrial depolarizations that occur after PVARP continues inthe third and fourth cycles. The pacing cycles of FIG. 11 illustratePMT.

The timing diagram illustrated in FIG. 12 illustrates PMT management inaccordance with embodiments of the invention. The first pacing cycleincludes a noncaptured atrial pace 1210 and a captured ventricular pace1230. The noncaptured atrial pace 1210 allows the depolarization causedby the ventricular pace to be retrogradely conducted to the atrium. Theretrograde conduction occurs after PVARP for the cycle has expired. Thenonrefractory atrial sense 1240 caused by the retrograde conduction isused by the CRM system to initiate a pacing cycle. The next ventricularpace 1231 is fast.

The CRM system initiates PMT management following the noncaptured atrialpace 1210 in the first cycle and the nonrefractory atrial sense 1240initiating the second cycle. The PVARP 1221 for the pacing cyclefollowing the noncaptured pace 1210, which is the second cycleillustrated in FIG. 12, is extended to break the PMT pattern. The nextatrial sense 1241 occurs in the extended PVARP 1221 and does notinitiate a pacing cycle. The third cardiac cycle illustrated in FIG. 12is a normal cycle.

The flowchart of FIG. 13 illustrates retrograde conduction managementand PMT management in accordance with embodiments of the invention. Anatrial and ventricular pace are delivered 1310 during a pacing cycle. Anatrial refractory period is timed 1320 for the pacing cycle. Arefractory or nonrefractory atrial depolarization is sensed 1330. If theatrial pace did not capture 1340 the atrium and the atrialdepolarization was sensed 1350 after expiration of the refractoryperiod, then PVARP is increased 1360 for one cardiac cycle. For example,the PVARP may be extended to about 500 ms. Extending the PVARP to 500 msfor one cardiac cycle breaks the PMT.

If the atrial pace did not capture 1340 the atrium and the atrialdepolarization was sensed 1350 during the refractory period, the systemchecks to determine 1370 if the time between the atrial depolarizationand the next scheduled atrial pace is greater than the tissue refractoryperiod (TRP). If not, the time for the next scheduled pace is extended1380 to avoid retrograde conduction in subsequent cardiac cycles. Forexample, the time for the next pace may be extended so that there isabout 300 ms between the refractory atrial sense and the next pace.

A number of the examples presented herein involve block diagramsillustrating functional blocks used for coordinated monitoring,diagnosis and/or therapy functions in accordance with embodiments of thepresent invention. It will be understood by those skilled in the artthat there exist many possible configurations in which these functionalblocks can be arranged and implemented. The examples depicted hereinprovide examples of possible functional arrangements used to implementthe approaches of the invention.

It is understood that the components and functionality depicted in thefigures and described herein can be implemented in hardware, software,or a combination of hardware and software. It is further understood thatthe components and functionality depicted as separate or discreteblocks/elements in the figures in general can be implemented incombination with other components and functionality, and that thedepiction of such components and functionality in individual or integralform is for purposes of clarity of explanation, and not of limitation.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. A method, comprising: delivering an atrial pacing pulse and aventricular pacing pulse during a first paced cardiac cycle; timing apost ventricular atrial refractory period (PVARP) following theventricular pacing pulse; determining if the atrial pacing pulsecaptures the atrium; scheduling delivery of a second atrial pacing pulsein a second cardiac cycle that occurs next after the first paced cardiaccycle; and in response to a determination that the atrial pacing pulsedid not capture the atrium: sensing an atrial depolarization occurringafter delivery of the first paced cardiac cycle; determining whether theatrial depolarization occurs during or after the PVARP; in response to adetermination that the atrial depolarization occurred during the PVARP,initiating retrograde management, to avoid retrograde conduction insubsequent cardiac cycles, by delaying a next scheduled atrial pacingpulse if the next scheduled atrial pacing pulse is scheduled to occurwhen the atrial tissue is refractory; and in response to a determinationthat the atrial depolarization occurred after the PVARP, initiatingpacemaker mediated tachyarrhythmia (PMT) management, to avoid PMT insubsequent cardiac cycles, by extending a subsequent PVARP of the secondcardiac cycle.
 2. The method of claim 1, wherein initiating theretrograde management comprises modifying pacing for at least one pacingcycle.
 3. The method of claim 1, wherein delaying the next scheduledatrial pacing pulse comprises delaying the next scheduled atrial pacingpulse until about 300 ms from the atrial depolarization.
 4. The methodof claim 1, wherein extending the subsequent PVARP comprises extendingthe subsequent PVARP to about 500 ms.
 5. The method of claim 1, whereinthe atrial depolarization comprises a retrogradely conducteddepolarization responsive to the ventricular pacing pulse.
 6. The methodof claim 1, wherein the atrial depolarization comprises a prematureatrial contraction.
 7. A cardiac rhythm management system, comprising: apulse generator configured to deliver an atrial pacing pulse and aventricular pacing pulse via electrodes coupled to a heart during afirst paced cardiac cycle; a post ventricular atrial refractory period(PVARP) timer configured to time a PVARP following the ventricularpacing pulse; a capture detector configured to determine if the atrialpacing pulse captures the atrium; a sensing system; a controller coupledto the capture detector and the sensing system, the controllerconfigured to: (a) in response to a determination that the atrial pacingpulse captured the atrium, schedule delivery of a second atrial pacingpulse in a second cardiac cycle that occurs next after the first pacedcardiac cycle; and (b) in response to a determination that the atrialpacing pulse did not capture the atrium, use the sensing system to sensean atrial depolarization occurring after delivery of the atrial andventricular pacing pulses, and determine whether such atrialdepolarization occurs during or after the PVARP, whereupon: in responseto a determination that the atrial depolarization occurred during thePVARP, using a retrograde management module to initiate retrogrademanagement pacing, to avoid retrograde conduction in subsequent cardiaccycles, by delaying a next scheduled atrial pacing pulse if the nextscheduled atrial pacing pulse is scheduled to occur when the atrialtissue is refractory; and in response to a determination that the atrialdepolarization occurred after the PVARP, using a pacemaker mediatedtachyarrhythmia (PMT) management module to initiate PMT management, toavoid PMT in subsequent cardiac cycles, by extending a subsequent PVARPof the second cardiac cycle.
 8. The system of claim 7, wherein theretrograde management module is configured to delay the next scheduleatrial pacing pulse until about 300 ms from the atrial depolarization.9. The system of claim 7, wherein extending the subsequent PVARPcomprises extending the subsequent PVARP to about 500 ms.
 10. The systemof claim 7, wherein the atrial depolarization comprises a retrogradelyconducted depolarization responsive to the ventricular pacing pulse. 11.The system of claim 7, wherein the atrial depolarization comprises apremature atrial contraction.
 12. A cardiac rhythm management system,comprising: a pulse generator configured to deliver an atrial pacingpulse and a ventricular pacing pulse via electrodes coupled to a heartduring a first paced cardiac cycle; a post ventricular atrial refractoryperiod (PVARP) timer configured to time a PVARP following theventricular pacing pulse; a capture detector configured to determine ifthe atrial pacing pulse captures the atrium; a sensing system; acontroller coupled to the capture detector and the sensing system, thecontroller configured with: (a) means for scheduling delivery of asecond atrial pacing pulse in a second cardiac cycle that occurs nextafter the first paced cardiac cycle, in response to a determination thatthe atrial pacing pulse captured the atrium; and (b) means for sensingan atrial depolarization occurring after delivery of the atrial andventricular pacing pulses and determining whether such atrialdepolarization occurs during or after the PVARP, in response to adetermination that the atrial pacing pulse did not capture the atrium,comprising: means for initiating retrograde management, to avoidretrograde conduction in subsequent cardiac cycles, by delaying a nextscheduled atrial pacing pulse if the next scheduled pacing pulse isscheduled to occur when the atrial tissue is refractory, in response toa determination that the atrial depolarization occurred during thePVARP; and means for initiating pacemaker mediated tachyarrhythmia (PMT)management, to avoid PMT in subsequent cardiac cycles, by extending asubsequent PVARP of the second cardiac cycle, in response to adetermination that the atrial depolarization occurred after the PVARP.13. The system of claim 12, further comprising means for delaying thenext scheduled atrial pacing pulse until about 300 ms from the atrialdepolarization.